At cryogenic temperatures a gas can be stored in a storage vessel in liquefied form to achieve a higher storage density, compared to the same gas stored in the gaseous phase. For example, higher storage density is desirable when the gas is employed as a fuel for a vehicle because the space available to store fuel on board a vehicle is normally limited.
The desired temperature for storing a liquefied gas depends upon the particular gas. For example, at atmospheric pressure, natural gas can be stored in liquefied form at a temperature of minus 160 degrees Celsius, and a lighter gas such as hydrogen can be stored at atmospheric pressure in liquefied form at a temperature of minus 253 degrees Celsius. As with any liquid, the boiling temperature for the liquefied gas can be raised by holding the liquefied gas at a higher pressure. The term “cryogenic temperature” is used herein to describe temperatures less than minus 100 degrees Celsius, at which a given gas can be stored in liquefied form at pressures less than 2 MPa (about 300 psig). To hold a liquefied gas at cryogenic temperatures, the storage vessel defines a thermally insulated cryogen space. Storage vessels for holding liquefied gases are known and a number of methods and associated apparatuses have been developed for removing liquefied gas from such storage vessels. The term “cryogenic fluid” is used herein to describe a fluid that is at a cryogenic temperature.
It is known to store a gaseous fuel, such as liquefied natural gas on board a vehicle for consumption by the vehicle's engine. Gaseous fuels such as, for example, natural gas, pure methane, hydrogen, and blends thereof are cleaner burning compared to conventional liquid fuels such as gasoline or diesel. Known approaches for consuming gaseous fuel on board a vehicle relate to introduction of the gaseous fuel into the engine's intake air manifold at relatively low pressures. For such applications, high-pressure cryogenic pumps are not needed, and at times the vapor pressure inside the storage vessel can alone be adequate, as taught by so-called economizer systems. For the relatively low pressures needed or desirable for gaseous fuels in commercially-known gaseous-fuelled engines, cryogenic pumps are not needed and are not used in conventional fuel systems that store a liquefied gaseous fuel at cryogenic temperatures. Recent developments have been directed to delivering a high-pressure gaseous fuel to an engine for injection directly into the combustion chamber; this approach enables the engine to emulate the performance and efficiency of a conventional diesel-cycle engine, which can be an improvement over known low-pressure fuel systems.
To supply a gaseous fuel to an engine at pressures high enough to allow direct injection into the combustion chamber, it is preferable to use a cryogenic pump to pump the liquefied gas, because this is more energy efficient compared to using a compressor to compress the fuel in the gaseous phase after the liquefied gaseous fuel has been vaporized. For example, for an engine that employs the same compression ratio as a conventional diesel engine, to inject a gaseous fuel directly into the combustion chamber, it is necessary or at least desirable to supply a gaseous fuel with a pressure that is substantially greater than the cylinder pressure at the time of injection. The required injection pressure is dependent upon the design of the engine. For example, to distinguish the presently disclosed method and apparatus from conventional “low pressure” gaseous fuel systems, for an engine used to power a vehicle like a truck, in order to inject a gaseous fuel directly into the engine's combustion chamber, a fuel injection pressure of at least about 20 MPa (about 3000 psi) is typically needed or at least desirable.
However, a problem with cryogenic pumps is that their performance can degrade over time for a variety of reasons. For example, if the cryogenic pump is a reciprocating piston pump, performance can degrade because of dynamic piston seal wear, and periodic maintenance is required to replace dynamic piston seals. The properties and consistency of quality of the process fluid that is being pumped is also a factor since impurities in the cryogenic fluid that are not caught by filters can accelerate the wear of dynamic piston seals and can also result in scoring of the pump cylinder. It is also possible that a manufacturing defect in a pump component can result in premature failure of the component and a consequent decline in pump performance. The manner in which a pump is operated and its duty cycle, including pump speed and frequency, can influence the longevity of wearing parts such as the dynamic seals. For example, in a vehicular application, the duty cycle can vary depending upon whether the vehicle is operated mostly on city streets or on-highway. Other causes of pump performance degradation can include static seal leakage, fitting leaking at connections between conduits and components, check valve leakage, or hydraulic system degradation or failure.
In a vehicular application, if cryogenic pump performance is allowed to degrade without diagnosing it and taking corrective action, vehicle performance can be affected, eventually forcing the vehicle to stop or be operated with a lower power output. Without a method of systematically diagnosing cryogenic pump performance, the only indication that something is wrong with the cryogenic pump is a change in vehicle performance and since there can be many other factors that influence vehicle performance, degraded pump performance can remain undetected until it is too late to take corrective action and the vehicle can be left stranded and/or damaged.
Accordingly, because there are a many potential causes of degradation in pump performance, that are unpredictable and that can affect pump performance before normal service intervals, there is a need for diagnosing cryogenic pump performance to determine pump performance over time, to detect when pump performance is degrading no matter what the cause, to establish appropriate service intervals, and to determine if a cryogenic pump requires maintenance between normal service intervals.